The current invention relates to methods for reducing the viscosity of certain high-viscosity fluid-loss control gels without increasing the likelihood of formation damage. Additionally, the current invention provides novel acid solutions for removing high-viscosity fluid-loss control gels from a subterranean formation. Accordingly, the current invention provides solutions and methods for restoring fluid permeability to subterranean formations that have been treated with chemical fluid loss control agents based on hydroxyethyl cellulose (HEC) polymers and derivatives prepared from HEC polymers. For the purposes of this disclosure, all such fluid loss control agents are referred to as HEC-based CFLA.
Key factors to achieving profitable production of oil and gas from subterranean formations include obtaining clear unobstructed perforations in well casing and maintaining an undisturbed formation behind the perforations. The ability to complete the process of perforating a well casing has improved to the point that large diameter clear holes in the casing are readily achieved. This advancement in hydrocarbon production has been achieved in part by the use of clear completion brines in place of mud products during perforation operations. However, the resulting immediate communication between the formation and the wellbore requires immediate control of fluid flow between the wellbore and the formation. If the density of the completion brine is too low, then the well may uncontrollably blow oil and gas up the casing. In contrast, if the brine density is too high, then the brine flows uncontrollably into the oil and gas bearing formation. Ideally, an absolute balance of wellbore fluid hydrostatic pressure against the pressure exerted by the fluids inside the formation would leave the well accessible and serviceable. Trying to maintain such a balance is dangerous and tricky due to the changing hydrostatic dynamics within deep petroleum wells. The safe option is always to make the wellbore brine heavy enough to overcome the formation pressure. However, achieving this balance increases the potential of detrimental interaction of the brine with the sensitive minerals of the formation. In fact, literature citing the need to limit the contact of brine with oil and gas bearing formations is readily available.
Common products employed to preclude the entry of brine into adjacent formations typically yield very high viscosity crosslinked gels in wellbore brines. Known as chemical fluid loss control agents (CFLA), the most common products utilize hydroxyethyl cellulose (HEC) polymer or chemically modified HEC polymer. In a typical application these gelled brine products, also known in the art as fluid loss control pills, reside in only a small area inside the wellbore adjacent to the perforations. In some cases the pills enter the perforations and the matrix of the perforated formation. Prior to initiating production from the formation, these fluid loss pills must be removed as completely as possible because they have the potential to restrict hydrocarbon production from the formation.
HEC-based CFLA responds to an acid environment by either uncrosslinking or by the polymer molecule breaking apart. The uncrosslinking or breaking lowers the viscosity of the gel pill. Following the reduction in viscosity, the HEC-based CFLA flows out of the formation thereby permitting fluid flow in either direction. In particular, a very effective acid used to achieve the uncrosslinking and breaking of the polymer gels is hydrochloric acid. Typically, the HCl is dissolved in brine to yield an acid concentration of 10% or less. Usually the HCl solutions completely remove the fluid loss pills in thirty to sixty minutes. Even though hydrochloric acid provides efficient and rapid removal of the fluid loss pills, the industry has reluctantly accepted the use of HCl solutions. A continued concern exists that the mineralogy of certain formations containing the zeolite family of clays responds to the presence of HCl by producing silica gel by-products. These by-products permanently damage the matrix of the formation where the fluid loss pill once resided. Therefore, a reluctance to use HCl acid solutions has emerged. Alternatively, the solutions have been so diluted as to be ineffective. Testing of alternative solutions such as organic acids and organic acid blends indicates that such solutions cause minimal formation damage; however, these solutions do not adequately remove all traces of HEC-based fluid loss polymer gels.
Therefore, a need exists for a breaker solution capable of providing the gel breaking ability of HCl acid solutions while substantially reducing the potential for by-product damage to the formation.